Conventionally, switching power supplies have been widely available as a power supply regulation circuit for a charger, each of which achieves a constant current drooping characteristic by means of a constant current control circuit provided on the secondary side. The switching power supply comprises, on the secondary side, a resistor for detecting an output current, a constant current control circuit for controlling current passing through the resistor to a constant level, and a photocoupler for transmitting the signal of the constant current control circuit to the primary side. When the output current is equal to or higher than a fixed value, the constant current control circuit is operated. According to the switching power supply having such a constant current drooping characteristic, batteries or the like can be charged with a constant current by using the constant current drooping characteristic.
Japanese Patent Laid-Open No. 2003-189612 proposes a switching power supply for achieving a constant current drooping characteristic without the provision of a constant current control circuit. The conventional switching power supply will be described below.
In the conventional switching power supply, a current limit dictating the maximum value of element current passing through a switching element is reduced as the output voltage reduces during a constant current operation, so that the constant current drooping characteristic is achieved.
FIG. 16 is a block diagram showing the configuration of the conventional switching power supply.
In FIG. 16, a control circuit 200 comprises, as input/output parts, a PIN terminal serving as a start-up power supply input part, a VCC terminal serving as an auxiliary power supply voltage input part, a CS terminal serving as a current detection/input part, an OUT terminal serving as a switching element driving/output part, and a GND terminal serving as the GND output part of the control circuit. Further, the control circuit 200 comprises an oscillator circuit. The control circuit 200 drives the gate of a switching element (MOSFET) 210 connected to the OUT terminal and controls the switching operation of the switching element 210.
The switching element 210 controls the on/off of current passing through the switching element 210 from a primary winding 220A of a transformer 220. Element current (drain current) ID passing through the switching element 210 forms a triangular wave which inclines in proportion to input voltage VIN due to the inductance of the primary winding 220A.
A rectifying/smoothing circuit comprising a diode 231 and a capacitor 232 converts alternating-current power, which is generated on a secondary winding 220B of the transformer 220 in response to the switching operation of the switching element 210, to direct-current power and supplies the power to a load 233.
A rectifying/smoothing circuit comprising a diode 241 and a capacitor 242 is used as the auxiliary power supply part and the output voltage detection part of the control circuit 200. The rectifying/smoothing circuit converts alternating-current power, which is generated on an auxiliary winging 220C of the transformer 220 in response to the switching operation of the switching element 210, to direct-current power and supplies the power to the VCC terminal through a resistor 243 and a capacitor 244. An alternating-current voltage generated on the auxiliary winging 220C is proportionate to an alternating-current voltage generated on the secondary winding 220B.
The control circuit 200 detects the drain current ID passing through the switching element 210, based on the voltage of the CS terminal fed with voltage generated on a resistor 250. The control circuit 200 has an overcurrent protection function for preventing overcurrent in the switching element 210. The overcurrent protection function is provided for automatically turning off the switching element 210 when a peak value Ip of the drain current ID increases to a current limit ILIMIT.
When the voltage of the VCC terminal (VCC voltage) is higher than a fixed voltage, the control circuit 200 reduces the on duty of the switching element 210 as the VCC voltage increases, so that a constant voltage characteristic is achieved.
Conversely when the VCC voltage is lower than the fixed voltage, the control circuit 200 reduces the current limit ILIMIT as current passing through the VCC terminal decreases, so that a constant current drooping characteristic is achieved. In this case, in order to keep an output current IO constant, the current limit ILIMIT is changed as a function of current passing through the VCC terminal.
Referring to the accompanying drawings, the following will describe the operations of the switching power supply thus configured. FIG. 17 is a timing chart showing the operations of the parts of the conventional switching power supply.
When the VCC voltage is higher than a fixed voltage VCC_A, the switching power supply reduces an on duty Don of the switching element 210 as the VCC voltage increases, so that a constant voltage characteristic is achieved.
When a load increases, the peak value Ip of the drain current ID passing through the switching element 210 is increased to the current limit ILIMIT, output power PO is maximized, and then the output voltage VO is reduced, the VCC voltage decreases in response to the reduction.
Thereafter, when the VCC voltage falls below the fixed voltage VCC_A, the switching power supply reduces the current limit ILIMIT according to a reduction of current passing through the VCC terminal, so that a constant current drooping characteristic is achieved. In this case, in order to keep the output current IO constant, the switching power supply changes the current limit ILIMIT as a function of current passing through the VCC terminal.
As described above, in the conventional switching power supply which achieves a constant current drooping characteristic without the need for a constant current control circuit, the maximum output power is determined by the current limit ILIMIT and an overload is detected.
In an actual circuit, however, the peak value Ip of the drain current ID passing through the switching element is higher than the current limit ILIMIT. This is because the switching element is not turned off immediately after the detection of the drain current ID having increased to the current limit ILIMIT. To be specific, a certain delay time called an overcurrent detection delay time Td is present between the detection of the drain current ID having increased to the current limit ILIMIT and the time when the switching element is actually turned off.
FIG. 18A shows the waveforms of the drain current ID when the current limit ILIMIT is constant relative to a given input voltage VIN. As described above, the drain current ID forms a triangular wave which inclines in proportion to the input voltage VIN. Meanwhile, the overcurrent detection delay time Td is constant. Thus when the current limit ILIMIT is constant relative to a given input voltage VIN, an increase in the current value of the drain current ID inevitably changes with the input voltage VIN in the overcurrent detection delay time Td. In other words, even when the current limit ILIMIT is constant, the peak value Ip of the drain current ID changes with the input voltage VIN. To be specific, as shown in FIG. 18A, a high input voltage VIN increases the inclination of the drain current ID and the peak value Ip of the drain current ID. A low input voltage VIN reduces the inclination of the drain current ID and the peak value Ip.
Therefore, in the case of a high input voltage VIN, the maximum value of the drain current ID is high as compared with a low input voltage VIN, and the maximum output power PO increases. This means that overload detection moves to the heavy load side. Thus, as shown in FIG. 19A, in the case of a high input voltage VIN, the output current IO in a constant current operation is high as compared with a low input voltage VIN. This relationship will be described below using the equation of the output power PO.
The output power PO in a discontinuous mode is expressed as below:PO=A×L×Ip2×foscwhere ‘fosc’ represents the oscillation frequency of the switching element, ‘A’ represents a constant, ‘L’ represents the inductance of the primary winding of the transformer, and Ip represents the peak value of the actual drain current ID.
As is evident from this equation, as long as the oscillation frequency fosc and the peak value Ip of the drain current ID passing through the switching element are constant, the output power PO remains constant even when the input voltage VIN changes. On the other hand, even when the oscillation frequency fosc remains constant, the output power PO changes with a change of the peak value Ip of the drain current ID. Therefore in the case where the current limit ILIMIT is constant relative to a given input voltage as described above, the maximum value of the drain current ID determined by the current limit ILIMIT changes with the input voltage VIN and thus the maximum output power PO also changes. Hence, as shown in FIG. 19A, in the case of a high input voltage VIN, overload detection moves to the heavy load side and the output current IO in a constant current operation is high as compared with a low input voltage VIN.
As described above, when the current limit ILIMIT is constant, the maximum value of the drain current ID changes with the input voltage VIN and the maximum output power PO also changes. Thus, the output current IO changes with the input voltage VIN in a constant current operation.
In order to solve this problem of the conventional switching power supply, the following technique is used: as shown in FIG. 20, in the conventional switching power supply, the current limit ILIMIT is linearly increased for a certain time after the switching element is turned on, and then the current limit ILIMIT is reduced to a value obtained when the switching element is turned on.
When the inclination of the increase of the sawtooth current limit ILIMIT is set at a proper value, as shown in FIG. 18B, a high input voltage VIN has a low current limit ILIMIT relative to the drain current ID, and a low input voltage VIN has a high current limit ILIMIT relative to the drain current ID. Therefore a change in the peak value Ip of the drain current ID according to the input voltage VIN decreases and thus a change in the output current IO also decreases in a constant current operation.
Strictly speaking, when the overcurrent detection delay time Td is constant, the current limit ILIMIT which keeps the peak value Ip of the drain current ID constant relative to a given input voltage VIN does not linearly change with time. In other words, it is necessary to change the current limit ILIMIT as expressed in the equation below:ILIMIT(t)=Ip×(t−Td)/twhere ‘t’ represents a time from when the switching element is turned on.
A current limit ILIMIT1 shown in FIG. 21 represents a current limit ILIMIT which sets, when the overcurrent detection delay time Td is 150 ns, the peak value Ip of the drain current ID at 1 A regardless of an input voltage. As shown in FIG. 21, the current limit ILIMIT1 is a function which simply increases and forms a convex shape with a primary time derivative acting as a positive derivative and a secondary time derivative acting as a negative derivative. The current limit ILIMIT1 does not linearly change with time.
On the other hand, a current limit ILIMIT2 is a current limit ILIMIT which linearly changes with time. The current limit ILIMIT2 is set such that the peak value Ip of the drain current ID has an error of about ±3% when the overcurrent detection delay time Td is 150 ns and an on time Ton of the switching element changes over a range of 1.0 to 4.5 μs. Further, a peak value Ip2 is an actual peak value Ip when the current limit ILIMIT2 serves as the current limit ILIMIT.
In this case, the on time Ton is a period during which the switching element is turned on. Strictly speaking, the on time Ton is a time during which the switching element is turned on, the drain current ID increases to the current limit ILIMIT, and then the switching element is turned off after a delay of the overcurrent detection delay time Td.
As shown in FIG. 21, even the current limit ILIMIT2 linearly changing relative to the on time Ton of the switching element can reduce the dependence of the peak value Ip of the drain current ID on the input voltage within a predetermined range of the on time Ton. The range of the on time Ton (1.0 to 4.5 μs) in this example is a 4.5-times range which can sufficiently respond to, for example, the input voltage range of world wide input (AC 85 to 282 V). Thus in the conventional switching power supply, the inclination of the increase of the current limit ILIMIT is set at a proper value, for example, like the current limit ILIMIT2 shown in FIG. 21.
However, in the case where the on time Ton is short relative to the overcurrent detection delay time Td, that is, “Td/Ton” is large, it is difficult to reduce the input voltage dependence of the peak value Ip of the drain current ID passing through the switching element. Thus the long overcurrent detection delay time Td and the short on time Ton are disadvantageous to a reduction of the input voltage dependence of the peak value Ip of the drain current ID. The input voltage dependence is reduced by the current limit ILIMIT linearly changing with time. This is proved from a fact that the peak value Ip2 of FIG. 21 is considerably deviated from 1 A at the on time Ton of 1 μs or less.
In the conventional switching power supply, as shown in FIG. 17, the current limit ILIMIT is reduced to supply lower energy to the secondary side, so that a constant current drooping characteristic is achieved. Therefore, in the conventional switching power supply, the peak value Ip of the drain current ID passing through the switching element is reduced and the on time Ton is shortened.
As described above, as the on time Ton becomes shorter, the peak value Ip of the drain current considerably changes. Thus in the conventional switching power supply, it is quite difficult to make the output current IO constant relative to a change of the input voltage VIN and a load.
FIG. 18C shows the waveforms of the drain current ID when the current limit ILIMIT decreases and the peak value Ip of the drain current ID passing through the switching element decreases.
As shown in FIG. 18C, in the case where the peak value Ip of the drain current ID decreases, the on time Ton of the switching element is shortened, and the inclination of the current limit ILIMIT is the same as the current limit ILIMIT shown in FIG. 18B, the peak value Ip of the drain current ID changes with the input voltage VIN. For this reason, in the conventional switching power supply which reduces the sawtooth current limit ILIMIT during a constant current operation and achieves a constant current drooping characteristic, it is expected that the constant current drooping characteristic is shaped like FIG. 19B. In other words, it is expected that the output current IO changes with the input voltage VIN when the output voltage VO decreases.